Mobile devices, for example mobile phones or smart phones, are frequently used in connection with portable hands free sets (PHF). These portable hands free sets comprise for example a stereo earphone, a microphone and an antenna. The portable hands free set is usually connected to the mobile device via a phone jack. The phone jack may comprise a multi-port plug-and-socket connection comprising for example four electrical contacts providing signal paths for a left and a right earphone signal, a microphone signal, and ground. An antenna signal may additionally be transferred by one of these signal paths, for example via a signal path of the left or the right earphone signal.
However, currently there are at least two different audio connector standards used for these phone jacks, the OMTP (open mobile terminal platform) used in Europe and the CTIA (cellular telecommunications industry association) used in North America. The two connector types differ at least in an interchanged position of the ground and microphone contacts. Therefore, a circuit design is need which allows to change the signals associated to the contacts to support both standards. However, due to the demand to use a common ground for the earphone audio signals and the microphone signals in combination with the demand of transferring the antenna signal, disturbances may occur especially on the microphone signal.
Therefore, there is a need to reduce disturbance on a microphone signal.